Rain pants might seem like a good idea for cycling, but in reality, they’re not very practical. Here’s why:

Overheating & Sweat – Most rain pants don’t breathe well, meaning you end up just as wet from sweat as you would from the rain.
Restricted Movement – They can be baggy and flap in the wind, making pedaling awkward and inefficient.
Water Seepage – Rain tends to run down from your jacket and into your shoes anyway, so keeping your legs completely dry is nearly impossible.
Bulk & Hassle – Putting them on and taking them off mid-ride is a pain, especially in unpredictable weather.
A much better option is cycling leg warmers. They keep you warm even when wet, dry quickly, and can be easily removed if needed. Combined with a good water-resistant jacket and overshoes, you’ll stay comfortable without sacrificing performance.

I agree with Matthew’s points that they easily become impractical.
Longjohns in merino wool are a very good garment in wet/cold conditions, and I have also tried cutting each leg off to and wore those, to make sure my butt/crotch area is directly on the bib.

I have however seen quite a few bikepackers in Norway using shell pants for the extra long and wet adventures. I think I would opt for a pair of hiking pants, forget about aerodynamics, and pack with me something along the lines of Vatne 3L Pant: https://www.bergans.com/no/p/1258-vatne-3l-pants

I would recomend Gore-Tex Packlight Plus rainshorts, instate of pants. With Merino legwarmers, you have a gut solution.

To some extent i agree with above comments but for long downpours i like my gore gtx paclite paints. As it has been shaped for cyclists, the fit is quite ok and without much flapping. What i also like is that it packs to a small volume with a weight of barely 200 gr. Comes at a cost though…

As with most things, it depends, in my opinion. I agree with Matthew in general, but in a heavy downpour I like to put on a pair of Goretex pants, preferrably one with zippers. In the summer, I usually bring with me a pair of Gore rainshorts, as mentioned above (https://www.gorewear.com/se-en/endure-gore-tex-shorts-101012?variant=13996) and I really like the convenience of these.

I can recommend the All Year Moab 2in1 rain pants from Vaude. These have ventilation slits on the side that can be easily opened with a zipper. Especially in changeable weather, this saves you having to take them on and off all the time.
Unfortunately, Vaude no longer has these in its range, but there are still a few left over here and there.

https://www.idealo.de/preisvergleich/OffersOfProduct/201690832_-vaude-mens-all-year-moab-2in1-rain-pants-vaude.html